1. Field of the Invention
This invention relates to digital communication systems and, more particularly, to correction of baseline wander in baseband transceiver systems.
2. Background of the Invention
The dramatic increase in desktop computing power driven by intranet-based operations and the increased demand for time-sensitive delivery between users has spurred development of high speed Ethernet local area networks (LANs). 100 BASE-TX Ethernet (see IEEE Std. 802.3u-1995 CSMA/CD Access Method, Type 100 Base-T) using existing category 5 (CAT-5) copper wire, and the newly developing 1000 BASE-T Ethernet (see IEEE Draft P802.3ab/D4.0 Physical Layer Specification for 1000 Mb/s Operation on Four Pairs of Category 5 or Better Twisted Pair Cable (1000 Base-T)) for Gigabit/s transfer of data over category 5 data grade copper wiring, require new techniques in high speed symbol processing. On category 5 cabling, gigabit per second transfer can be accomplished utilizing four twisted pairs and a 125 megasymbol/s transfer rate on each pair where each symbol represents two bits.
Physically, data is transferred using a set of voltage pulses where each voltage represents one or more bits of data. Each voltage in the set is referred to as a symbol and the whole set of voltages is referred to as a symbol alphabet.
One well-known system of transferring data at high rates is Non Return to Zero (NRZ) signaling. In binary NRZ signaling, the symbol alphabet {A} is {xe2x88x921, +1}. A logical xe2x80x9c1xe2x80x9d is transmitted as a positive voltage while a logical xe2x80x9c0xe2x80x9d is transmitted as a negative voltage. At 125 M symbols/s, the pulse width of each symbol (the positive or negative voltage) is 8 ns.
An alternative well-known modulation method for high speed symbol transfer is MLT3 and involves a three level system. (See American National Standard Information system, Fibre Distributed Data Interface (FDDI)xe2x80x94Part: Token Ring Twisted Pair Physical Layer Medium Dependent (TP-PMD), ANSI X3.263:1995). The symbol alphabet for MLT3 is {A}={xe2x88x921, 0, +1}. In MLT3 transmission, a logical 1 is transmitted by either a xe2x88x921 or a +1 while a logic 0 is transmitted as a 0. A transmission of two consecutive logic xe2x80x9c1xe2x80x9ds does not require the system to pass through zero in the transition. A transmission of the logical sequence (xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d) would result in transmission of the symbols (+1, 0, xe2x88x921) or (xe2x88x921, 0, +1), depending on the symbols transmitted prior to this sequence. If the symbol transmitted immediately prior to the sequence was a +1, then the symbols (+1, 0, xe2x88x921) are transmitted. If the symbol transmitted before this sequence was a xe2x88x921, then the symbols (xe2x88x921, 0, +1) are transmitted. If the symbol transmitted immediately before this sequence was a 0, then the first symbol of the sequence transmitted will be a +1 if the previous logical xe2x80x9c1xe2x80x9d was transmitted as a xe2x88x921 and will be a xe2x88x921 if the previous logical xe2x80x9c1xe2x80x9d was transmitted as a +1. The actual voltage levels that are transmitted are typically +1 V, 0 V and xe2x88x921 V for the +1 symbol, the 0 symbol and the xe2x88x921 symbol, respectively.
The detection system in the MLT3 standard, however, needs to distinguish between 3 levels, instead of two levels in a more typical two level system. The signal to noise ratio required to achieve a particular bit error rate is higher for MLT3 signaling than for two level systems. The advantage of the MLT3 system, however, is that the energy spectrum of the emitted radiation from the MLT3 system is concentrated at lower frequencies and therefore more easily meets FCC radiation emission standards for transmission over twisted pair cables. Other communication systems may use a symbol alphabet having more than two voltage levels in the physical layer in order to transmit multiple bits of data using each individual symbol. In Gigabit Ethernet over twisted pair CAT-5 cabling, for example, 5-level pulse amplitude modulated (PAM) data with partial response shaping is transmitted at a baud rate of 125 Mbaud. (See IEEE Draft P802.3ab/D4.0 Physical Layer Specification for 1000 Mb/s Operation on Four Pairs of Category 5 or Better Twisted Pair Cable (1000 Base-T)).
FIG. 1A shows a typical transmission system 100 for transmitting data at high rates over conventional twisted copper pair wiring. Transmission system 100 includes a transmitter 101, a transmit coupler 102, a transmission channel 103, a receive coupler 104 and a receiver 105. The transmitter 101 receives data in the form of a symbol stream from a host 111 through a medium independent interface (MII) 112 and couples the modulated data into transmission medium 103 through transmit coupler 102. Receive coupler 104 receives a modulated waveform from transmission medium 103 and couples the modulated waveform into receiver 105. The modulated waveform received in receiver 105 suffers from the effects of intersymbol interference (ISI) caused by channel distortion, transmit and receiver filters in transmitter 101 and receiver 105, and couplers 102 and 104. Receiver 105 outputs the received data, after correcting for channel distortion, to host 113, via a medium independent interface 114.
Intersymbol interference can be compensated for by equalization in receiver 105. However, some of the effects resulting from couplers 102 and 104, which are typically transformers, are not compensated adequately by equalization in receiver 105. These effects include baseline wander and killer packets.
Baseline wander refers to the result of a transmission, in baseband transceiver systems, of symbols where most of the symbols are of identical polarity, for example, in MLT-3 transmission a long series of ones or negative ones. In that case, the output signal from transmitter 101 appears to be a DC signal (a constant 1 V is transmitted by transmitter 101 if a long series of +1 symbols is transmitted). In general, the baseline of the transmit signal is shifted up or down based on the polarity of the transmitted data. Couplers 102 and 104 are typically inductors and, therefore, do not pass DC voltages. The net effect is that the input signal to receiver 105 suffers an exponential decay, called droop or xe2x80x9cbaseline wanderxe2x80x9d, eventually resulting in increased error rates in the receiver if the baseline wander effect is not adequately compensated.
In addition, some particular data sequences result in peak-to-peak voltage levels at the receiver that are much higher than other data sequences. For example, even though transmitter 101 outputs a signal having a peak-to-peak voltage of 2 V, because of the effects of couplers 102 and 104 the input signal at receiver 105 can be as high as about 4 V peak-to-peak in response to certain sequences of symbols. A sequence of transmitted symbols that results in particularly high peak-to-peak voltages at receiver 105 is referred to as a xe2x80x9ckiller packet.xe2x80x9d An example of a killer packet satisfying the transmission constraints of a 100 BaseTX system is given in American National Standard for Information Systems, ANSI X3.263:1995, Fibre Distribued Data Interface (FDDI)xe2x80x94Part: Token Ring Twisted Pair Physical Layer Medium Dependent (TP-PMD), March 1995.
In order to process symbol streams that include killer packets, analog-to-digital converters in receiver 105 are required to receive even the statistically less likely, but higher voltage level, signals that result from such packets. This results in either an increased cost for analog-to-digital conversion (i.e., utilization of higher resolution analog-to-digital converters), a loss of resolution of the receiver detection circuitry by setting the resolution of the analog-to-digital converter low enough to include the higher range of voltages, or allowing the analog-to-digital converter to clip the input signal resulting from killer packets. All of the above solutions are, therefore, undesirable.
Corrections for baseline wander and receipt of killer packets have depended on a model of the transformer and have been implemented, at least partially, with analog circuitry. FIG. 1B shows a correction circuit that is commonly used. Receiver 105 receives signals from a transmission channel 110. The signals from transmission channel 110 include distortion from filters in transmitter 101 (FIG. 1A), filters in receiver 105, intersymbol interference (ISI) from the transport medium, and the effects of couplers 102 and 104. The signal is corrected for the effects of the couplers 102 and 104 in adder 106 and equalized in equalizer 107 (FIG. 1B). Slicer 108 receives the signals from equalizer 107 and decides on an output symbol stream.
The output symbol stream is received by transformer modeler 109 that executes a transfer function that corrects for the effects of couplers 102 and 104. The transfer function includes corrections for baseline wander and for receipt of killer packets. This approach to correction, usually accomplished in an analog circuit, depends on the transformer and relies on the transformer model transfer function being accurate. The correction, i.e. adder 106, is accomplished before any analog-to-digital conversion of the signal, resulting in the need for analog circuitry or a digital-to-analog converter if the correction is calculated digitally. The analog implementation usually defeats the higher reliability and increased economic savings of a digital signal processing implementation.
Therefore, a receiver that digitally corrects for baseline wander and that is independent of the actual coupling transformers is desirable. In addition, a receiver that receives xe2x80x9ckillerxe2x80x9d packets without a subsequent loss of resolution for the analog-to-digital conversion, without using a more expensive analog-to-digital converter, and without resorting to an analog implementation of a correction circuit is desirable.
In accordance with the invention, a receiver of a communications system includes a digital baseline wander circuit. In one embodiment, the receiver includes an analog-to-digital converter, coupled to receive signals from a transport channel and outputting a sample, and a slicer that receives the sample and outputs a symbol. The baseline wander circuit receives the output symbol from the slicer and the input sample to the slicer, digitally executes a transfer function that outputs a baseline correction estimate, and corrects the output sample of the analog-to-digital converter using the baseline correction estimate.
The baseline wander circuit, according to this invention, is implemented digitally and is responsive to the input signals of the receiver. The implementation does not depend on a model of the coupling transformers used to couple the transmitter and the receiver to the transport media.
In some embodiments, an equalizer is coupled between the analog-to-digital converter and the slicer. The equalizer can include a linear equalizer or a decision feedback equalizer. The baseline wander circuit corrects the output of the analog-to-digital converter before the signal is received by the equalizer.
Another embodiment of the invention includes the ability to receive killer packets. The receiver includes an A/D reference voltage circuit that, based on an indication of the cable length, adjusts the reference voltage of the A/D converter of the receiver in preparation for receiving the higher peak-to-peak voltages of killer packets. In one embodiment, the gain from a gain control circuit indicates the cable length. In other embodiments, adaptively chosen equalizer parameters can be utilized to indicate the cable length.
These embodiments are further discussed below in relation to the following figures.